Passive radio frequency identification (RFID) is a non-contact automatic identification technology, which can automatically identify a target object and acquire related data through a radio frequency signal without need of human intervention and can be applied to various harsh environments. The RFID technology can identify high-speed moving objects and simultaneously identify a plurality of tags, and be operated rapidly and conveniently.
The RFID technology adopts the following basic operating principle: a tag, after entering into a magnetic field, receives radio frequency signal from a reader, and sends product information (a passive tag) stored in a tag chip by means of energy acquired through induced current, or actively sends a signal at a certain frequency (a active tag); the reader reads and decodes the signal, and sends the decoded signal to an central information system for corresponding data processing.
In the passive RFID technology, the energy that a tag need for working is extracted from a signal of a reader. Therefore, certain measures are required to be taken for increasing the energy acquired at the tag end during the process that information transmits from the reader to the tag; one of the common method is to increase the time that signal operates from the reader to the tag, and another method is to use high level as much as possible and reduce the use of low level in a link from the reader to the tag during a data encoding process. For example, in the 18000-6 Type C of international organization for standardization (ISO), a pulse interval encoding (PIE) method is adopted.
The Manchester encoding method is a widely applied self-clocking coding method, which has the advantages of abundant timing and synchronization information carried during an encoding process, no DC drift, and simple encoding. The Manchester encoding method replaces each old binary code respectively with two new binary codes with different phases and is suitable for information transmission from a reader to a tag during radio frequency identification.
One of the encoding principles of the Manchester encoding method is as follows:
From 0 to 01 (a zero-phase periodic square wave);
From 1 to 10 (a π-phase periodic square wave).
During a specific implementation of the Manchester encoding method, data are distinguished by the change of wave electrical level; when the level changing from low to high, it indicates binary data “0”; when the level changing from high to low, it indicates binary data “1”; and a jump happens at the medium stage of a symbol period.
However, when the Manchester encoding method is used for transmitting data to a passive tag, the energy acquired by the passive tag is somewhat low.